A railway rail tends to distort longitudinally (i.e. along its length) during the cooling from rolling temperature thereof, because of the asymmetric cross section of the typical railroad rail. Distortion is increased if there exists an uneven application of heat or cooling to different portions of the rail cross-section. Such longitudinal distortion is referred to as camber. There are a number of different types of camber, including up sweep, down sweep, or up-and- down sweep. "Up-sweep" is the out of straight condition that exists when a railroad rail is placed head up on a horizontal support and, as a result, such rail has ends that are higher than its middle. "Down-sweep" is the opposite of this condition. "Up-and-down sweep" is a combination of the above two conditions in the same rail. Side sweep is another form of longitudinal distortion not of concern during most heat treatment processes, since rails are symmetric about their vertical axis and the desired pattern of hardening is also usually symmetric about such axis.
The longitudinal shape of a rail may be controlled during an accelerated cooling process by the use of a restraining system wherein a plurality of rollers are used to maintain the longitudinal shape of the rail. Any such restraint system, however, will have certain drawbacks. The exertion of external force onto the rail can induce residual stress which, in turn, increases the possibility of rail failure in service. Secondly, a mechanical restraint system may introduce mechanical defects into the rail. These mechanical defects would lead to higher defect rates and if undetected, could become fracture initiation sources in use.
Apparatus for reducing rail camber caused by intermittently cooling the head portion of a hot railroad rail is disclosed by Ackert et al. in U.S. Pat. Nos. 4,486,248 and 4,611,789. Such apparatus includes means for cooling the bottom of the rail base, but not the tips thereof, and a roller system designed to restrain and transport the rail in a head-up longitudinally straight position. The rail bottom cooling means comprises means for spraying a liquid cooling medium onto the base bottom, to help balance thermal contraction and stresses associated with the metallurgical transformations occuring during forced cooling. However, the base of the rail can only undergo a limited amount of cooling without increasing the toe hardness to unacceptable levels, because cooling the base from the bottom draws heat from the toe of the rail base. The toes of rails are subject to rapid cooling (because of the large surface to volume ratio) and adding coolant to the base increases the heat sink seen by the toe of the rail base.
Another prior art method, disclosed in U.S. Pat. No. 4,668,308 to Economopoulos et al., involves cooling the entire periphery of the rail or cooling the web and base. The disadvantage of cooling the web completely is the loss of heat available for soak back into the head of the rail as described by Ackert et al. This lack of soak back would necessitate complex process control to produce consistently fine pearlite without encountering bainite or martensite. These complexities are avoided, by allowing uniform soak back from the upper rail web to the web-head junction, in the present invention.